PROJECT SUMMARY/ABSTRACT Twist1, a transcriptional factor with upregulated expression in various cancers, plays important roles in cancer development and metastasis. However, the exact mechanistic pathways by which Twist1 regulates carcinogen-specific lung cancer initiation and progression remain largely unknown. Therefore, new insight into how lung cancer initiates and progresses mediated by Twist1 is urgently needed. Our preliminary data suggest that overexpression of Twist1 is frequently found in lung cancer and correlated to its prognosis, Twist1 overexpression could be carcinogen-specific and related to its underlying molecular changes, and inhibition of Twist1 could lead to the decrease of tumorigenicity of lung cancer cells. In the current application, we propose to do the followings: 1) Evaluate the role of Twist1 in carcinogen-specific lung carcinogenesis using in vitro and in vivo models. (a) Evaluate the involvement of Twist1 in cellular response to BaP or NNK in lung epithelial cells and (b) Assess the importance of Twist1 alterations in the tumor development in BaP or NNK-induced lung cancer mouse model. 2) Identify the Twist1-mediated molecular mechanisms in carcinogen-specific lung tumorigenesis. (a) Evaluate the regulation of Twist1 cellular localization by shuttling factors (G3BP2, p62, and exportins) and Twist1-interacting proteins after BaP or NNK exposure and (b) Determine Twist1-mediated signaling molecules including p53, K-ras, and EGFR after BaP or NNK exposure. 3) Investigate the clinical significance of targeting Twist1 for lung cancer treatment. (a) Evaluate the combinative effect of Twist1 and the EGFR TKI in cell lines and the xenograft mouse model and (b) Examine the anti-drug resistance of targeting Twist1 for the EGFR TKI in cell lines and the xenograft mouse model. These studies will help improve our understanding of carcinogen-specific molecular progression of lung carcinogenesis mediated by Twist1, and identify the clinical significance of targeting Twist1 for lung cancer treatment.